The present invention relates to a liquid crystal display and a fabrication method thereof. More particularly, the present invention relates to a liquid crystal display which provides a wide viewing angle and a high-speed response and a fabrication method thereof.
FIGS. 23(a)(b) are side cross-sectional views showing an operation of a liquid crystal in the conventional liquid crystal panel and FIGS. 23(c)(d) are front views thereof. In FIG. 23, an active element is omitted. Here, part of one pixel is shown, although a plurality of pixels are formed by stripe-shaped electrodes.
FIG. 23(a) is a side cross-sectional view showing a cell in the state in which no voltage is applied and FIG. 23(c) is a front view thereof. Line-shaped electrodes 103, 104 are formed on an inner surface of one of a pair of transparent substrates and an alignment control layer 106 is applied thereon and has been subjected to alignment treatment. A liquid crystal composition is interposed between the substrates. While no voltage is applied, bar-shaped liquid crystal molecules 105 are oriented to have a certain angle with respect to the longitudinal direction of a stripe-shaped Y electrode, i.e., 45 degrees xe2x89xa6|xcfx86Lc| less than 90 degrees (xcfx86Lc: an angle with respect to a long axis (optical axis) of liquid crystal molecules in the vicinity of interfaces). Here, it is assumed that an orientation direction of the liquid crystal molecules on upper and lower interfaces is parallel, and dielectric anisotropy of the liquid crystal composition is positive.
Subsequently, when an electric field 109 is applied, as shown in FIGS. 23(b), (d), the liquid crystal changes its direction according to the electric field. By placing a polarizer 102 at a predetermined angle 108, light transmittance can be varied by application of a voltage. Thus, display with contrast is realized without a transparent electrode.
However, in such an In-Plane Switching type liquid crystal display, a response of a nematic liquid crystal to the electric field is slow. In addition, as shown in FIG. 23, since the electrode has a peculiar structure such as the strip shape and it is difficult that the electric field is applied to the liquid crystal, the response speed is low.
Rise time xcfx84rise and Fall time xcfx84fall of the liquid crystal of the In-Plane Switching type are represented by the following expressions disclosed in Japanese Laid-Open Patent Publication No. Hei. 7-225388:
xcfx84rise=xcex31/(xcex50xcex94xcex5E2xe2x88x92xcfx802K2/d2)xe2x80x83xe2x80x83(1) 
xcfx84fall=xcex31d2/xcfx802K2=xcex31/xcex50xcex94xcex5E2xe2x80x83xe2x80x83(2) 
where xcex31 is viscosity coefficient, K2 is elastic constant of twist, d is cell gap, xcex94xcex5 is dielectric anisotropy, xcex50 is vacuum dielectric constant, E is electric field strength, and Ec is threshold electric field.
In order to achieve the high-speed response in the In-Plane Switching type liquid crystal display, according to the first and second expressions, the cell gap disreduced, a liquid crystal material with low viscosity coefficient and high dielectric constant (e.g., cyano-based liquid crystal material) is employed, or a drive voltage is increased to increase an electric field strength E.
However, the following problems remains unsolved in the above-described liquid crystal display.
(1) If the cell gap is reduced, then time required for filling the liquid crystal and hence, time for fabrication are increased. Also, uneveness due to a variation in gap precision is noticeably observed.
(2) When a cyano-based liquid crystal material is used instead of a fluorine-based liquid crystal material or an addition ratio thereof is increased, heat resistance and light resistance become unstable, which might bring about inferior display such as partial abnormality in contrast or flicker.
(3) If the drive voltage is increased, a consumed power is correspondingly increased. Besides, a drive IC which has been conventionally used cannot be used and there is a need for a dedicated drive ID.
(4) If the transparent electrode such as ITO is used as a pixel electrode or a common electrode for the purpose of improvement of the transmittance and response speed, it is necessary to form a layer with larger thickness. But, if the layer is formed so as to be thicker, transmittance is reduced and layer surface is roughened because fine crystals are deposited. For this reason, a light diffusing value is increased and light availability is reduced.
FIG. 24 is a cross-sectional view showing an In-Plane Switching type liquid crystal display disclosed in Japanese laid-Open Patent Publication No. Hei. 9-236820. As defined herein, the In-Plane Switching is a method in which a pixel electrode and a counter electrode are both formed on an inner surface of one of transparent substrates, potential is given across the pixel electrode and the counter electrode on the same plane, and a lateral electric field parallel to planes in which the transparent substrates respectively exist is applied to the liquid crystal, thereby controlling arrangement of liquid crystal molecules. This method is directed to improving display viewing angle dependency of the display.
FIG. 24(a) shows a cross section in the direction orthogonal to a source bas line (video signal line) and in the vertical direction (direction orthogonal to the substrate surface) of a portion including no semiconductor switching element mentioned later. FIG. 24(b) shows a cross section of the portion including the semiconductor switching element. FIG. 24(c) shows a cross section of the portion in the direction parallel to the source bas line and including the semiconductor switching element.
In FIG. 24, 201a denotes a lower transparent substrate and 201b denotes an upper transparent electrode. 202 denotes a counter electrode. 203a denotes a gate electrode. 204 denotes a source bas line. 205 denotes a pixel electrode and 205a denotes its extended end portion. 206 denotes a semiconductor switching element. 207 denotes a liquid crystal layer. 208a denotes a lower alignment layer and 208b denotes an upper alignment layer. 209 denotes a transparent insulating layer.
As shown in FIG. 24, in this liquid crystal display, the two transparent substrates 201a, 201b are placed opposite to each other, the liquid crystal is filled between opposed surfaces thereof, and alignment layers in contact with the upper and lower surfaces of the liquid crystal layer are adapted to align the liquid crystal molecules to have a predetermined orientation, which has been conventionally adopted.
The characteristics of this display are as follows. A transparent insulating layer 209 is placed between the alignment layer 208a and the transparent substrate 201a on the side of the array substrate, i.e., the transparent substrate on which the electrode is formed, which corresponds to the substrate 201a in this display. The transparent insulating layer serves to insulate between the source bas line and the counter electrode and between the source bas line and the pixel electrode. In addition, the counter electrode and the source bas line can overlap with each other when seen from a user of this display (when the user of this display sees a display screen).
This constitution enables an area of a light-blocking portion due to the existence of the electrode to be reduced and an aperture ratio of the pixel portion to be increased. Consequently, luminance of the entire screen is improved.
However, the following problems exists in this liquid crystal display.
(1) When the pixel electrode and the counter electrode formed in the pixel portion are comprised of non-transparent conductive layers, light is not transmitted through these electrodes, thereby causing the aperture ratio to be decreased. On the other hand, even if the pixel electrode and the counter electrode are comprised of the transparent conductive layers, the electric field strength over the electrodes is small when the conventional electrode constitution and liquid crystal material are employed. Since little light is transmitted, a substantial improvement of the aperture ratio is unfulfilled.
(2) By forming the gate electrode (scanning signal line) and the counter electrode through the same process, the fabrication process can be simplified. Nevertheless, since the respective electrodes are in close proximity with each other, they are electrically shortened, thereby causing a reduced yield ratio.
(3) The counter electrode located right above the source bas line (video signal line) affects an electric field distribution formed by most of counter electrodes other than the counter electrode provided right above the source bas line (video signal line) and the pixel electrode.
(4) Since the electric field distribution formed by the counter electrode located right above the source bas line and the pixel electrode differs from the electric field distribution formed by the other counter electrodes and pixel electrodes, this causes the luminance non uniformity or coloring.
An object of the present invention is to provide a liquid crystal display which provides a wide viewing angle, a high speed response, and a high image quality such as high luminance without changing a liquid crystal material, reducing cell gap, or increasing a drive voltage, and a fabrication method thereof.
To achieve the above-described object, the invention according to claim 1 is a liquid crystal display comprising an array substrate in which a common electrode, a pixel electrode, a scanning signal line, a video signal line, and a semiconductor switching element are formed, an opposed substrate, and a liquid crystal layer interposed between the array substrate and the opposed substrate, wherein a voltage is applied across the pixel electrode and the common electrode to generate an electric field substantially parallel to the substrates to thereby drive the liquid crystal to control light, characterized in that line width of at least one of the common electrode and the pixel electrode is larger than gap between the common electrode and the pixel electrode.
By setting the line width of the electrode larger than the gap between the common electrode and the pixel electrode (electrode spacing), the electrode spacing is made substantially smaller. As a result, since the rising of the electric field spreads to the electrode inner side and the electric field strength in the vicinity of the the end of the electrodes is increased, the response speed is improved.
In the present invention, film thickness of at least one of the common electrode and the pixel electrode may be larger than film thickness of at least one of the scanning signal line and the video signal line.
Also, in this way, since the rising of the electric field spreads to the electrode inner side, response speed is improved.
In the present invention, line width of at least one of the common electrode and the pixel electrode may be larger than gap between the common electrode and the pixel electrode, and film thickness of at least one of the common electrode and the pixel electrode may be larger than film thickness of at least one of the scanning signal line and the video signal line.
This constitution provides a synergetic effect greater than the effect obtained by addition of the effect resulting from increasing of the line width and the effect resulting from increasing of the film thickness.
In the present invention, at least one of the common electrode and the pixel electrode may be comprised of a transparent conductive layer.
As described above, by reducing the electrode spacing or increasing the thickness of the electrode, the electric field spread to the region over the electrodes. Therefore, by using the transparent electrode instead of the non-transparent electrode, the portions located right above the electrodes can be used as a display portion. Consequently, transmittance can be improved.
In the present invention, at least one of the common electrode and the pixel electrode may be comprised of at least two types of conductive layers.
In the present invention, at least one of the common electrode and the pixel electrode may be comprised of transparent conductors having at least two types of different optical characteristics.
In the present invention, at least one of the common electrode and the pixel electrode may be comprised of an insulating layer and a conductive layer formed on a surface thereof.
In the present invention, at least one of the common electrode and the pixel electrode may be comprised of a transparent insulating layer and a transparent conductive layer formed on a surface thereof.
In the present invention, electrode gap between the common electrode and the pixel electrode may be at least smaller than gap between the array substrate and the opposed substrate.
In the present invention, part of or all of the common electrode and the pixel electrode may be comprised of amorphous transparent conductive film. The film thickness of the transparent conductive film may be 1500 xc3x85 or more.
At least one of the array substrate and the opposed substrate may be made of resin.
The present invention is a method for fabricating a liquid crystal display in which at least part of a common electrode and a pixel electrode is comprised of an amorphous transparent conductive film, characterized in that the transparent conductive film is formed at 100xc2x0 or less.
The transparent conductive film may be formed by adding H2O or H2 and without heating.
The present invention is a liquid crystal display comprising an array substrate in which a common electrode, a pixel electrode, a scanning signal line, a video signal line, and a semiconductor switching element are formed, an opposed substrate, and a liquid crystal layer interposed between the array substrate and the opposed substrate, wherein a voltage is applied across the pixel electrode and the common electrode to generate an electric field substantially parallel to the substrates to thereby drive the liquid crystal to control light, characterized in that at least one of the common electrode and the pixel electrode is comprised of a wiring portion and an electrode portion formed in different layers with an insulating layer interposed therebetween, and the electrode portion is comprised of a transparent conductive layer.
By thus forming the electrode portion using the transparent conductive layer, the transmittance can be made higher than that of the conventional example in the liquid crystal display with layered pixel electrode and common electrode.
The wiring portion of the common electrode may be formed in the same process as the scanning signal line. The wiring portion of the pixel electrode may be formed in the same process as the video signal line.
One of or both of line width of each of electrode portions constituting the common electrode and the pixel electrode and spacing between the electrode portion of the common electrode and the electrode portion of the pixel electrode may be substantially equal to or smaller than gap between the array substrate and the opposed substrate.
Also, line widths of the electrode portions comprised of transparent conductive layers are not limited to the line widths of the electrode portions formed using the non-transparent conductive layers and may have different values. In particular, it is preferable that line widths of the electrode portions comprised of transparent conductive layers are larger than line widths of the electrode portions comprised of non-transparent conductive layers. In this way, the transmittance can be increased.
The present invention is a liquid crystal display comprising an array substrate in which a common electrode, a pixel electrode, a scanning signal line, a video signal line, and a semiconductor switching element are formed, an opposed substrate, and a liquid crystal layer interposed between the array substrate and the opposed substrate, wherein a voltage is applied across the pixel electrode and the common electrode to generate an electric field substantially parallel to the substrates to thereby drive the liquid crystal to control light, characterized in that part of the common electrode and the video signal line are a layered common electrode and a layered video signal line which are layered with an insulating layer interposed therebetween such that patterned positions thereof are overlapped with each other when seen from a direction orthogonal to a surface of the substrate, and one of or both of line width of each of electrode portions constituting the common electrode and the pixel electrode and gap between the electrode portion of the common electrode and the electrode portion of the pixel electrode is substantially equal to or smaller than gap between the array substrate and the opposed substrate.
In the present invention, at least one of the layered electrodes may be comprised of the wiring portion and the electrode portion and the electrode portion may be comprised of a transparent conductive layer.
The layered electrode may be comprised of the wiring portion and the electrode portion, and the wiring portion may be formed in the same process as the scanning signal line or the video signal line.
Also, a layer including the electrode portion of the layered common electrode that is located right above the layered video signal line may be comprised of a non-transparent conductive layer.
Also, a layer including the electrode portion of the layered common electrode that is located right above the layered video signal line may be different from a layer in which the other common electrodes are formed.
At least one of the layered common electrode and the layered pixel electrode may be formed on the insulating layer on the side of the array substrate.
The present invention is a liquid crystal display comprising an array substrate in which a common electrode, a pixel electrode, a scanning signal line, a video signal line, and a semiconductor switching element are formed, an opposed substrate, and a liquid crystal layer interposed between the array substrate and the opposed substrate, wherein a voltage is applied across the pixel electrode and the common electrode to generate an electric field substantially parallel to the substrates to thereby drive the liquid crystal to control light, characterized in that part of the common electrode and the video signal line are a layered common electrode and a layered video signal line which are layered with an insulating layer interposed therebetween such that patterned positions thereof are overlapped with each other when seen from a direction orthogonal to a surface of the substrate and line width of the electrode portion of the layered common electrode that is located right above the layered video signal line is different from line width of the electrode portions of the other common electrodes.
In the present invention, the line width of each of the electrode portions constituting the common electrode and the pixel electrode may allow liquid crystal molecules over the electrode portion comprised of the transparent conductive layer to be modulated by an electric field generated between the common electrode and the pixel electrode.
The line width of each of the electrode portions constituting the common electrode and the pixel electrode may be less than twice of gap between the array substrate and the opposed substrate.
The line width of each of the electrode portions constituting the common electrode and the pixel electrode may be 3 xcexcm to 8 xcexcm.
The dielectric constant anisotropy xcex94xcex5 of a liquid crystal material of the liquid crystal layer may be +8 or more. Bend elastic constant K33 of a liquid crystal material of the liquid crystal layer may be 18 (pN) or less. Further, retardation xcex94nxc3x97d of the liquid crystal layer may be 200 nm-600 nm.
The liquid crystal layer may be made of a liquid crystal material having a 35% or less content of the cyano-based compound.
The electrode portions constituting the common electrode and the pixel electrode may be bent so as to include at least one bent portion in a pixel. The video signal line may be bent so as to have a bending angle substantially equal to that of a bending shape of the electrode portions constituting the common electrode and the pixel electrode.
The semiconductor switching element may be a channel etched thin film transistor.
Part of the semiconductor switching element may be made of polysilicon.
The present invention is a method for fabricating a liquid crystal display comprising the steps of: forming a non-transparent conductor on an active matrix substrate and patterning a first electrode group comprised of part or all of a common electrode and a scanning signal line to have a predetermined shape; forming a first insulating layer on the active matrix substrate having the first electrode group; forming a semiconductor layer on a predetermined portion of the first insulating layer; forming a non-transparent conductor on the first insulating layer and the semiconductor layer and patterning a second electrode group comprised of part or all of a video signal line and a pixel electrode to have a predetermined shape; forming a second insulating layer on the active matrix substrate that has been subjected to the step of forming the second electrode group and its previous steps, and forming a transparent conductor on the second insulating layer and patterning a third electrode group comprised of part of the common electrode and/or part of the pixel electrode to have a predetermined shape.
Also, the present invention is a method for fabricating a liquid crystal display comprising the steps of: forming a non-transparent conductor on an active matrix substrate and patterning a first electrode group comprised of part or all of a common electrode and a scanning signal line to have a predetermined shape, forming a first insulating layer on the active matrix substrate having the first electrode group; forming a semiconductor layer on a predetermined portion of the first insulating layer; forming a non-transparent conductor on the first insulating layer and the semiconductor layer and patterning a second electrode group comprised of part of or all of a video signal line and a pixel electrode to have a predetermined shape; forming a second insulating film on the active matrix substrate that has been subjected to the step of forming the second electrode group and its previous steps; forming a transparent conductor on the second insulating film and patterning a third electrode group comprised of part of one of the common electrode and the pixel electrode; forming a third insulating layer on the active matrix substrate that has been subjected to the step of forming the third electrode group and its previous steps; and forming a transparent conductor on the third insulating layer and patterning a fourth electrode group comprised of part of the other of the common electrode and the pixel electrode group to have a predetermined pattern.
The method for fabricating the liquid crystal display may further comprise the step of forming a third insulating layer subsequent to a step of forming the third electrode group to have the predetermined pattern.
The method may further comprise the step of forming a fourth insulating layer subsequent to a step of forming the fourth electrode group to have a predetermined pattern.
In the method for fabricating the liquid crystal display, the third insulating layer or the fourth insulating layer may be made of SiNx-based material, or a photosensitive resin material or an SiO2-based material.
It is preferable that Sb2O5-based fine particles may be added to the SiO2-based material.